Ceramic matrix/ceramic fiber composites having a thin-walled shape, such as a tube, shell, etc., have been fabricated by chemical vapor infiltration (CVI) wherein a fibrous preform is heated isothermally and exposed to a reactive vapor which includes gaseous chemical precursors for the ceramic matrix for long periods of time (e.g. a week or more). During this time, the gaseous chemical precursors infiltrate the fibrous preform and react to deposit the ceramic matrix material on the surfaces of the individual preform fibers and thereby form the matrix of the composite. The rate of the matrix-forming reaction is governed by the temperature of the preform and gaseous reactants, the concentrations of reactants, and the concentration of byproduct gases released by the reaction.
An illustrative reaction for depositing a SiC matrix would involve hydrogen and methyltrichlorosilane (MTS), CH.sub.3 SiCl.sub.3, as gaseous reactants. At high temperature (e.g. greater than 700.degree. C.), these reactants can deposit SiC on the fiber surfaces with HCl being a byproduct gas that is released by the reaction and that inhibits the deposition of SiC on the fiber surfaces.
In this particular isothermal CVI process, the concentration of MTS is maximum at the preform outer surface and diminishes toward the interior of the preform, while the concentration of the HCl byproduct gas increases towards the interior of the preform. Both of these effects result in preferential deposition of SiC near the outer surface of the fiber preform. Eventually, the surface porosity of the preform is sealed by the deposited SiC, leaving a core region of the preform inadequately infiltrated with the ceramic matrix. One attempt to minimize these adverse effects has involved conducting the CVI process at relatively low temperatures using diluted reactant gases. However, even after very long times (e.g. on the order of 2 months), it has been virtually impossible to make a ceramic/ceramic composite with a cross-section thicker than about 12 mm (millimeters). Even then, the resulting composite microstructure exhibits undesirable porosity.
Another attempt to minimize these adverse affects has involved establishing a temperature gradient across the fiber preform from one side of the preform to the other so that matrix deposition occurs preferentially at the hotter preform side and the cooler preform surface porosity remains open for a longer duration throughout the CVI process. For example, work at Oak Ridge National Laboratory has demonstrated that ceramic matrix/ceramix fiber composites can be made relatively rapidly (e.g. on the order of one day) by maintaining a sharp temperature gradient through the preform to provide a relatively cold side and a relatively hot side while concurrently imposing a flow of gaseous reactants through the preform in the direction of the increasing temperature; i.e. flowing the reactant gases from the cooler preform side to the hotter preform side. By this temperature gradient/forced parallel reactant flow CVI technique, the ceramic matrix is deposited preferentially near the hot side of the preform such that the zone of deposition moves from the hot side toward the colder side as the CVI process is continued. However, this technique is limited to making ceramic/ceramic composites having very simple geometries, such as flat plates and tubes as a result of the need for the temperature gradient across the preform.
It is an object of the invention to provide an improved method/apparatus for making a composite wherein a matrix is microwave-assisted chemical vapor deposited from the interior toward the exterior of reinforcement or filler material confined in a microwave-transparent, shaping envelope in a manner that permits a wide variety of composite sizes (large and small) and configurations (simple and complex) to be made in a relatively rapid time.
It is another object of the invention to provide an improved method/apparatus for making a surface composite on a monolithic body using microwave-assisted chemical vapor deposition from the interior toward the exterior of reinforcement or filler material disposed on the monolithic body.
It is still a further object of the invention to provide a composite wherein the composite includes a matrix chemical vapor deposited from the interior toward the exterior of the reinforcement or filler material of the composite as a result of microwave heating of the reinforcement or filler material during infiltration.
It is still a further object of the invention to provide a composite including relatively thick and thin sections that are microwave-assisted, chemical vapor infiltrated with a matrix material.
It is still another object of the invention to provide a composite wherein the composite is confined in a microwave-transparent envelope that imparts a desired shape to the composite.